The present invention relates to the field of resistor-based memory circuits. More particularly, it relates to a method for precisely sensing the resistance value of a resistor-based memory cell, for example, a Magnetic Random Access Memory (MRAM) magnetic memory cell.
A resistor-based memory such as a magnetic random access memory (MRAM) typically includes an array of resistor-based magnetic memory cells. The logic state of such a magnetic memory cell is indicated by its resistance. One resistance value, e.g., the higher value, may be used to signify a logic high while another resistance value, e.g., the lower value, may be used to signify a logic low. The value stored in each memory cell can be determined by measuring the resistance value of the cell to determine whether the cell corresponds to a logic high or low. Such direct measurements are often difficult to simply and easily implement and require a number of comparators which increases the cost and size of the memory circuit. A simplified, more reliable method of sensing the resistance value of a resistor-based memory cell is desired.
The present invention provides a simple and reliable method and apparatus for sensing the logic state of a resistor-based memory cell. Resistance is measured by first charging a first capacitor to a predetermined voltage, discharging the first capacitor through a resistance to be measured while discharging a second capacitor through an associated reference resistance of known value and comparing the discharge characteristics e.g. the discharge voltage of two capacitors to determine a value of resistance to be measured relative to the reference resistance.
In one exemplary embodiment, a pair of second capacitors are used, each discharging through an associated reference resistance, one having a value corresponding to one possible resistance value of the resistance to be measured and the other having a value corresponding to another possible resistance value of the resistance to be measured. The combined discharge characteristics of the pair of second capacitors, e.g. an average of the discharge capacitor voltage, is compared with the discharge characteristics e.g. the discharge voltage of the first capacitor to determine a value of the resistance to be measured relative to an average value of the two reference resistances.